Scale reducing heating element for water heaters

Abstract

An extended life electrical heating element for a water heater includes a coiled heating resistance wire having a uniform power output per coil turn. Where the heating resistance wire passes through the sheath at critical areas, e.g. return bends, the number of coil turns per unit length of element is reduced to reduce thermal power output per unit length of the element. The number of coil turns per unit length of element in bend areas may be reduced by simply stretching the coiled heating wire to attain the desired length of resistance wire per unit length of the element. Resistance wires of differing heat output per unit length may be combined with different degrees of stretching to achieve the desired element temperatures. The reduced power at the bend portions reduces element temperature to reduce hard water scaling and significantly extend the life of the element.

What is claimed is: 
     
       1. An electrical heating element for a water heater, comprising: an elongate sheath of a heating element, said sheath having first and second ends and a bend portion and non-bend portions between said ends;   a helically coiled resistance heating wire having first and second ends, said wire enclosed within said sheath and extending between said first and second ends of said sheath;   means for connecting said first and second ends of said heating wire to an external electrical source for generating heat; and   the length of heating wire per unit length of sheath in the bend portion comprises a first value, the length of heating wire per unit length of sheath in a non-bend portion comprises a second value, and the first value being substantially less than the second value.   
     
     
       2. The electrical heating element of claim 1, wherein said ratio of the first value to the second value is between about 0.1 and about 0.9. 
     
     
       3. The electrical heating element of claim 1, wherein the ratio of the first value to the second value is between about 0.2 and about 0.8. 
     
     
       4. The electrical heating element of claim 1, wherein said second value is variable. 
     
     
       5. The heating element of claim 1, wherein said non-bend portions of said sheath comprise generally parallel straight portions joined by a bend portion. 
     
     
       6. The heating element of claim 1, wherein said sheath includes a first straight portion joined to a second straight portion by a first bend portion, a third straight portion joined to a fourth straight portion by a second bend portion, and a third bend portion joining said second and fourth straight portions, said first through fourth straight portions being generally parallel. 
     
     
       7. The heating element of claim 1, further comprising an electrically insulating, heat conducting material surrounding said heating wire within said sheath. 
     
     
       8. The heating element of claim 1, wherein said first and second ends of said element are proximate each other and are sealably joined to an element wall mount. 
     
     
       9. An electrical water heater, comprising: a generally cylindrical pressure vessel having a wall with a sealable port;   an electrical heating element assembly having an elongate element and an element mount sealably attached to said element, said mount positioned within said port to seal said port, whereby said element comprises: an elongate sheath of a heating element, said sheath having first and second ends and a bend portion and non-bend portions between said ends;   a helically coiled resistance heating wire having first and second ends, said wire enclosed within said sheath and extending between said first and second ends of said sheath;   means connected to said first and second ends of said sheath for sealably inserting said heating element through the wall of a water heater into the interior thereof; and   means for connecting said first and second ends of said heating wire to an external electrical source for generating heat;   whereby the length of heating wire per unit length of sheath in the bend portion comprises a first value, the length of heating wire per unit length of sheath in a non-bend portion comprises a second value, and the ratio of said first value to said second value is about 0.1-0.9.     
     
     
       10. The electrical water heater of claim 9, wherein the ratio of said first value to said second value is about 0.2-0.8. 
     
     
       11. A resistance heating wire for an electrical heating element having straight portions, at least one bend portion and two ends proximate each other and joined to an element mount, comprising: a continuously coiled heating wire preformed to provide a high density of coil turns in straight portions of a heating element and a lower density of coil turns in a bend portion of the heating element, wherein the density of coil turns in said bend portion is 0.1-0.9 that in the straight portions.   
     
     
       12. The resistance heating wire of claim 11, wherein the density of coil turns in said bend portion is 0.2-0.8 that in the straight portions. 
     
     
       13. An electrical heating element for a water heater, comprising: an elongate sheath of a heating element, said sheath having first and second ends and at least two bend portions between said ends, said at least two bend portions differing in radii of curvature;   a helically coiled resistance heating wire having first and second ends, said wire enclosed within said sheath and extending generally between said first and second ends of said sheath;   means for connecting said first and second ends of said heating wire to an external electrical source for generating heat; and   the heating wire is preconfigured by selective stretching along spaced apart sections of its length prior to being enclosed within the sheath and is positioned within the sheath to provide a lesser length of heating wire per unit length of sheath in the bend portion where the radius of curvature is lesser.   
     
     
       14. The electrical heating element of claim 13, wherein the sheath has a first bend A having a radius of curvature R 1  and a second bend B having a radius of curvature R 2 , wherein the heating wire length L 1 , L 2  per unit sheath length for said bend A and said bend B, respectively are related by:   L.sub.2 /L.sub.1 =(R.sub.2 /R.sub.1).sup.2,     where s is a scaling factor equal to 0.05 to 0.75.   
     
     
       15. The electrical heating element of claim 14, wherein the scaling factor is equal to a number between 0.1 and 0.65. 
     
     
       16. The electrical heating element of claim 15, wherein the scaling factor is equal to a number between 0.15 and 0.6. 
     
     
       17. An electrical heating element for a water heater, comprising: an elongate sheath of a heating element, said sheath having first and second ends and at least two bend portions between said ends, said at least two bend portions differing in radii of curvature;   a resistance heating wire having first and second ends, said wire enclosed within said sheath and extending generally between said first and second ends of said sheath;   means for connecting said first and second ends of said heating wire to an external electrical source for generating heat; and   the resistance heating wire being constructed and preconfigured prior to being enclosed within the sheath to comprise length portions of lower and high heat outputs per unit length, said portions of wire having lower heat output per unit length being within said bend portions of lower radius of curvature and said portions of wire having higher heat output per unit length being within said bend portions of higher radius of curvature.   
     
     
       18. The electrical heating element of claim 17, wherein the sheath has a first bend A having a radius of curvature R 1  and contains a heating wire portion of length L 1  having a heat output H 1  per unit length, said sheath containing a second bend B having a radius of curvature R 2  and containing a heating wire portion of length L 2  having a heat output H 2  per unit length, wherein the heating wire length L 1 , L 2  per unit sheath length for said bend A and said bend B, respectively are related by:   (L.sub.2 ×H.sub.2)/(L.sub.1 ×H.sub.1)=(R.sub.2 /R.sub.1).sup.s,     where s is a scaling factor equal to 0.05 to 0.75.   
     
     
       19. The electrical heating element of claim 18, wherein the scaling factor is equal to a number between 0.1 and 0.65. 
     
     
       20. The electrical heating element of claim 19, wherein the scaling factor is equal to a number between 0.15 and 0.6.